Best places: Philadelphia/NYC area, Central Florida, Southern California, England, Germany, France, Japan
Philadelphia/NYC area: Dorney, Hershey, Knoebels, Morey's Piers, SF America, SF Great Adventure, and many smaller parks within driving distance for a day trip
Central Florida: Disney World, Universal, Sea World, and Busch Gardens all within 2 hours of each other
Southern California: Disneyland, Knott's, SF Magic Mountain, and many fun centers within 2 hours of each other
England: Many parks and easy to get to using public transport
Germany, France, and Japan: See England
Worst places: Anywhere west of I-35 (outside of Central and Southern California), Houston, Louisiana, Upper Peninsula of Michigan, Northern New England, Canada (outside of Toronto and Montreal)
Anywhere west of I-35: Many states west of I-35 have very few if any roller coasters
Houston: The fourth-largest city in the United States has only one roller coaster within 2 hours, thanks to Six Flags and Hurricane Katrina
Louisiana: Hurricane Katrina, enough said
UP of Michigan and Northern New England: Nearest roller coasters over four hours away and very little freeway to get there
Canada: Only two decent-sized amusement parks in entire country (Canada's Wonderland and La Ronde)